The present disclosure relates generally to navigation and geolocation and, more particularly, to beacon-based geolocation using a low frequency electromagnetic field, such as a ULF (ultra low frequency) or VLF (very low frequency) electromagnetic field.
Geological mapping and geophysical surveying on the Earth's surface are mature sciences with a history of technology enhancements that improved the fidelity of understanding of the Earth, above and beneath the surface. Yet, when conventional techniques are employed in an underground environment, geolocation has proven a challenge.
Conventional mapping and survey systems, such as a Global Positioning System (GPS), determine the location of objects using satellite signals. However, a longstanding problem exists with determining location of personnel and equipment within, for example, underground facilities without the use of surveying. To date, this problem has not been resolved because of the difficulty of signaling/communicating between the Earth's surface and underground facilities/caverns/mines and the complexity of electromagnetic propagation within the Earth.
Lower fidelity, VLF systems are currently in development to support communications for cave rescue operations. These systems only obtain a shallow depth position when the communication system is used underground. In addition, VLF communications systems are effective up to about 600 meters (m) and occasionally up to about 1200 m. The VLF systems are also used to locate underground transmitters at comparable depths. In controlled experiments, they have achieved an accuracy of 2% in horizontal position and only 5% in depth.
A typical manner of providing time base synchronization between a transmitter and receiver used for navigation purposes has been to either (1) provide a uniform time radio reference signal from an independent source (e.g., GPS or VLF signal) or (2) provide each transmitter and receiver with its own highly accurate and stable timing mechanism which are then mutually synchronized at the beginning of the period of interest. However, in underground environments, GPS and VLF signals are either unavailable or unreliable. Providing each device with its own stable time base may be expensive, cumbersome, and wasteful of limited available power supplies.